Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation

Abstract

Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO 2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Because of the ultrafast time scale of the ET reaction of about 6 fs, the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations, and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the generation of phonons during polaron formation in the TiO 2 lattice is observed in real time for this system. The presented investigations enable an unambiguous assignment of the involved photoinduced mechanisms and can contribute to a corresponding extension of molecular ET theories to ultrafast ET systems like alizarin/TiO 2 . ■ INTRODUCTIONMolecular electron transfer (ET) belongs to the most important and ubiquitously encountered processes in the fields of chemistry and biology. The standard theoretical treatment for molecular ET was developed by Marcus 1,2 providing a correlation of the difference in Gibb's free energy (ΔG) between donor and acceptor, the curvature of the potential energy surfaces (PES), and the reorganization energy (λ). Within the framework of this nonadiabatic theory, ET rates of many systems were predicted quite well. Further quantum mechanical extensions by theories of Hopfield 3 and Jortner 4 were then even able to further extend the scope of theoretical ET models. All of these ET theories are based on the assumption that the energies of the donor and the acceptor states are matched by energy fluctuations caused by the surrounding solvent acting as thermal bath. In a standard photoinduced ET reaction a bimolecular donor (D)−acceptor (A) system is excited from the ground state DA into the Franck−Condon region of a higher electronic state DA* (Figure 1a). In a classical view the system can subsequently propagate along the PES of the electronic configuration DA*, approximated as one-dimensional parabola. Energy conservation allows the electronic transition between the states DA* and D + A − only at the intersection of the two PESs. In the picture of electronic quantum dots the propagation of the system on the PES of the state DA* leads to a periodic modulation of the electronic energy level of the donor (Figure 1a, inset) corresponding to the potential energy in the PES picture. In the configuration where the energies of donor and acceptor levels match (i.e., exactly at the crossing point of the DA* and the D + A − parabolas), ET can occur as a tunneling process with a certain probability, depending on the electronic coupling matrix element between the state DA* and the charge-separated state D + A − . Based on this microscopic view of ET, macroscopic rates can be calculated by assuming a thermal occupation of the energy eigenstates of the DA* potential. Thus, within the Marcus picture, t...

Conformational and Binding Effects on Interfacial Electron Transfer from Dual-Linker Sensitizers

“…To ensure that the modulation did not arise from artifacts, this result was reproduced on 6 different samples, including different batches of colloidal films and in 21 individual measurements. Such dynamics has been observed in a small number of IET systems and can be explained by the formation of a vibrational wavepacket in the excited state of the sensitizer. − The coherent wavepacket is triggered by the short excitation pulse. It modulates IET, giving rise to enhanced IET whenever the wavepacket approaches the transition between the donor potential energy surface and the acceptor states in the conduction band of TiO 2 .…”

Lifetime Broadening and Impulsive Generation of Vibrational Coherence Triggered by Ultrafast Electron Transfer

“…This explains why no vibrational wavepacket motion is observed with I, IIa and IIIa which undergo slower CS. The impulsive generation of coherent vibrational wavepackets upon an ultrafast ET was predicted theoretically, 33 and was reported for heterogeneous photoinduced ET at a chromophore-semiconductor interface, 34,35 for photoinduced ET in a electron-donating solvent, 32 for photoisomerisation in liquid, 36 as well as for singlet fission in a crystal. 37 However, to the best of our knowledge, it has not been observed so far for an intramolecular CS process in liquid.…”

Ultrafast photoinduced energy and charge transfer

“…With the advent of ultrafast (fs-ps) spectroscopy at the end of the 1980s, it became possible to track in "real-time" EET and ET processes in biological [80][81][82][83][84][85][86][87][88][89] and chemical systems [90][91][92] and at interfaces, 11,58,60,93,94 using pump-probe techniques. Vibrational coherences generated by ultrafast EET and ET have also been reported in various systems 90,[95][96][97][98][99] and a nice discussion of such processes and of the debates around them are presented in the paper by Wu et al (DOI: 10.1039/ c8fd00190a) of this issue. In general, ultrafast pump-probe spectroscopy iden-ties these coherences from an a priori knowledge of frequencies and modes, but electronic coherences that would be characterised by similar frequencies as the vibrational ones makes the two difficult to distinguish.…”